Electrical transmission and distribution systems of the common overhead type include a plurality of electrical conductors. The electrical conductors are typically bare and are supported by insulating means attached to a pole or other suitable structure to suspend or support the conductors a safe distance above the reach of normal ground traffic and machinery. A variety of connectors are employed in the construction of such systems including splices, dead-ends, taps and terminals. Of these, splices and dead-end connectors are utilized in tension applications. Such connectors provide not only electrical continuity to conductors joined thereby, but also the mechanical means to support the conductor under tension to traverse the span between support structures.
A variety of conductor configurations exist and the two principal types of conductors utilized in the modern electrical grid are copper and aluminum. Some common types of conductors are stranded aluminum alloy conductors (AAC), all aluminum alloy conductors (AAAC) and aluminum conductors steel reinforced (ACSR). The conductors and connectors used in tensioned electrical transmission systems and lines have a finite electrical and mechanical service life. For example, the electrical interface of all conductors and connectors is subject to a variety of aging phenomena, which serve to degrade the interface and increase the electrical resistance thereof over time. This aging effect can be accelerated by many factors, such as increased operation temperature, improper installation and adverse environmental conditions.
A reasonable service life for aluminum connectors of the type used in overhead distribution and transmission applications, when properly assembled with the appropriate inhibitor and operated within their thermal design limitations, is proving to be approximately 40 to 60 years. However, aluminum connectors that are operated in corrosive environments, at elevated thermal levels or those improperly installed, tend to fail in less than 25 years, sometimes in as little as 15 years, and often in as little as 5 years.
The use of aluminum conductors and connectors in the electrical power grid became prevalent during World War II, when shortages of copper forced utilities to seek alternate means to transfer electric current. Engineering breakthroughs resulted in superior conductors based on aluminum stranding, utilizing hard drawn alloys, tempered alloys, and composite conductor constructions having steel cores to reinforce their tensile properties. At the end of the war, an improved economy and technology fueled a major focus on construction, resulting in electrification of the greater parts of Europe and North America, as well as other parts of the world. Thus, the major portions of the electrical grid in place today were built between the late 1940's and the early 1970's, the results being that the majority of our existing electrical infrastructure is 35 to 60 years old. As previously stated, a reasonable service life for aluminum connectors used in overhead distribution and transmission is proving to be approximately 40 to 60 years, while the conductor remains usable for possibly another 20 years or more. Currently, the connectors are beginning to fail at an alarming rate.
The advent of increasing power demands in recent decades combined with the construction of new power lines lagging severely behind the construction of a new generation of new homes, businesses and industries has resulted in operating the existing grid at an ever increasing electrical current load. Consequently, these higher electrical current levels result in much higher conductor and connector temperatures. This increase in electrical load on the transmission and distribution infrastructure serves to amplify the current density and thermal stress on the entire system. These are just some of the factors that are serving to accelerate the inevitable failure of millions of electrical connectors that are already in the latter stages of their service life.
Additionally, a number of these aluminum bodied tension connectors have particular thermal limitations, typically about 93° C. When the thermal limitation is exceeded, the tempered aluminum alloys anneal, resulting in a loss of tensile properties on an order of about 65% to 70% of their original ultimate design strength. A great number of these types of connectors have already failed catastrophically due to operation of the line beyond their design limits.
Connectors that serve both as mechanical tension anchors and electrical connectors are particularly prone to fail catastrophically. The failure of such connectors results in energized power lines falling into and onto the general public, power outages and in some cases, property damage or severe personal injury and death.
One option is to construct new power transmission and distribution systems. Another option is to replace the old conductors and connectors with new ones that operate at temperatures as high as 250° C. However, right of way for new structures has become increasingly difficult or impossible to obtain, and replacement of existing conductors and connectors is not economically justifiable when the existing conductor still may have 20 to 30 years of usable life.
As to failing connectors, one option is to replace the connectors. This is an extremely expensive undertaking. The process typically includes interrupting power, cutting out the failed connector and replacing it with two new connectors and a length of conductor. In some instances, to avoid using two new connectors and an additional length of conductor, a single extended length replacement connector is employed. Installation of the single extended length connector is also an expensive and time consuming undertaking.
Furthermore, on critical service lines where an interruption cannot be tolerated, an electrical jumper must be attached, followed by attachment of a mechanical device to support the conductor while the replacement process is performed on the energized conductor. This is even more expensive, typically priced in the thousands of dollars per connector, and is obviously extremely dangerous.
Another possibility is to build a shunt system around a connector by utilizing two tee type tap connectors attached to the conductor on each respective end of the failing connector. A jumper is then attached between these tap connectors. While this addresses the electrical interface, it does not address the weakened mechanical condition of the connector. Thus, there would still be a significant risk of mechanical failure.